A tachometer is often used for measuring velocity and operates much like a generator. For example, when a motor drives the tachometer, the tachometer generates an output voltage. The produced output voltage is proportional to the velocity of the motor. The velocity signal may then be fed to a rate indicator to display the velocity to the operator, or it can be used to close a velocity loop. More particularly, the tachometer drive circuit receives pulses from an ignition coil in the ignition system of the motor and then displays these pulses as engine speed.
One method by which tachometer or ignition trigger signals may be generated is by inductively coupling a magnetic sensing pickup on the ignition coil high voltage spark output wire. This method has been used in ignition timing lights for decades. U.S. Pat. No. 6,058,902 to Jacobs discloses a circuit in which the original vehicle ignition coil high-tension lead is connected to a sense resistor to sense the ignition trigger for an after-market ignition trigger input. Other methods include tapping into the ignition coil negative lead to monitor the coil kickback signal when the coil is fired on an inductive ignition coil to generate the tachometer signal. A disadvantage of these methods is their inability to produce a clean tachometer output signal using simple, reliable, low cost circuitry.
Further, a particularly significant limitation of the prior art methods described above is their inability to extract a clean accurate tachometer signal from an ignition coil that is in multi-spark operation. As such, regardless of the method used to detect coil operation, the multi-spark signal must be managed to provide a proper tachometer output signal. Another limitation is that in an engine having multiple coils, each of the coils must be connected to the tachometer signal generating circuit, resulting in the need for up to eight inputs on a coil per plug ignition system. Furthermore, extra wires are often needed for providing battery voltage to the signal generating circuit. In addition, many of the existing methods do not have the capability to be used universally in all existing single and multiple ignition coil equipped vehicles.